The number of credit hours one is taking.
Difficulty also plays a role, though, practically speaking - lab classes, although they may be only 1, or no, hours, often are more difficult by far than the accompanying course.

This semester, I had a lab class that took only 2 hours a week, but consumed more of my time than all my other classes combined. The fact that I was learning Java, an entirely new programming language to me, was a big part of that.